We studied the effect of protein-bound polysaccharide PSK on metastatic colonization of BALB/c mice after intravenous injections of different syngeneic murine H-2 positive and H-2 negative tumor clones. The tumor lines used were different clones from chemically induced fibrosarcomas (GR9.B9, an H-2 negative clone from GR9 tumor, and B7.1.B4, an H-2 positive clone from B7.1 tumor). These clones were selected because of their different sensitivity to NK cytotoxicity, which was related to MHC class I expression. Pretreatment of mice with PSK inhibited metastatic colonization derived from B9 H-2 negative tumor cells. In contrast, lung colonization of PSK treated mice injected with B7.1.B4 H-2 positive tumor cells was higher, and differences in the number of colonies between untreated and PSK treated mice were small. In several experiments the effect of PSK was attenuated to a greater degree when high numbers of cells were injected. Abrogation of NK cells with anti-asialo GM1 serum significantly increased (in all tumors and at different cell doses) the number of metastatic colonies in comparison with untreated mice injected with tumors, regardless of the cell dose used. These results clearly suggest that NK cell activation in vivo by the protein bound polysaccharide PSK abrogates metastasis formation in mice. Abrogation was dependent on the H-2 phenotype even when pretreatment consisted of a single dose of PSK. This effect, related to the NK sensitivity of the tumor target, can be used to predict the effect of PSK in vivo.